In fiber optics communications, Coarse Wavelength Division Multiplex CWDM transmission standard has been defined to achieve low cost performances in metro networks.
The ITU-T G694.2 standard defines a grid for CWDM systems consisting in 20 nm spaced signals from 1270 nm wavelength on. In those applications the span lengths are limited principally by the fiber losses, the attenuation profile of which meets the absorption OH— peak around 1390 nm (about 3 dB/km). Even at few channels below or above the two attenuation minima at 1310 nm and 1550 nm, the system performances are deteriorated.
For those reasons the span length of a CWDM (or more generally WDM) system may be limited by the channel having the worst insertion loss. The existing implementation approach of CWDM is derived from the general WDM communication technique developed in the erbium optical amplifier band (typically 1530–1565 nm), that is a symmetical structure for the tap order of mux and demux optics, normally of the pass-band thin-film 3-port passive optical filter type, where the tap order of mux and demux are inverted.
The known solution takes care of maintaining the S/N ratio as high as possible in the optical amplifiers chain. This target can't be maintained in a CWDM network for many reasons.